http://www.surfacetreatments.it/thinfilms Plasma Etching of Niobium surfaces: Studies on samples and Single-Cell Cavities (AnneMarie Valente - 30') Speaker: AnneMarie Valente - Jefferson Lab - Newport News (VA) USA | Duration: 30 min. Abstract Plasma based surface modification provides an excellent opportunity to eliminate impurities and defects in the penetration depth region of Nb SRF cavity surfaces. It also allows a better control of the final SRF surface as final surface modifications like oxidation or nitridation can be done in the same process cycle. In the framework of a collaboration between ODU and Jefferson Lab, we are pursuing the use of environmentally friendly dry etching of SRF cavity in an Ar/Cl2 discharge. The experimental conditions in the microwave glow discharge system with a barrel-type reactor have been optimized. The viability of plasma etching as an alternative surface preparation method for bulk Nb surfaces has been demonstrated on flat samples by achieving etching rates comparable to wet processes, such as BCP or EP. The optimized experimental conditions are now being applied to the preparation of single cell cavities. The geometry of SRF cavities made of bulk Nb defines the use of asymmetric RF discharge configuration for plasma etching. The asymmetry in the surface area of a driven and grounded electrode creates a difference in the voltage drop over the plasma sheath attached to the driven electrode and the plasma sheath attached to the cavity surface. A specially designed single cell cavity with sample holders is used to study these asymmetric discharges. The sample holder ports can be used for both diagnostics and sample etching purposes. The approach is to combine radially and spectrally resolved profiles of optical intensity of the discharge with direct etched surface diagnostics to obtain an optimum combination of etching rates, roughness and homogeneity in a variety of discharge types, conditions and sequences.

1. Anne-Marie Valente-Feliciano for JanardanUpadhyay Jefferson Lab

4. Outline Motivation Work plan Plasma etching on Flat Samples Single Cell Cavity Experiment Concluding remarks

6. Plasma-assisted etch process (dry etching) is the enabling process in semiconductor industry, since it can be highly selective with respect to direction and hence indispensable in patterned removal of surface material or in removal of material from non-flat surfaces [2].(b)

8. A variety of surfaces can be intentionally created through plasma processing

9. Pure Nb2O5, or other cap layer

10. superconducting NbN

12. Phase 2: Work with a single-cell cavity to establish optimum conditions for an asymmetric electronegative discharge in cavity geometry, to demonstrate the uniformity of surface treatment, and to perform the RF performance test compatible with existing standards, to establish treatment protocol, and to define the process monitoring procedure environmentally.

14. Flat Samples - Microwave Glow Discharge System

15. Flat Samples - Etching Rate Dependence on Discharge Parameters M. Rašković, S. Popović, J. Upadhyay, and L. Vušković, L. Phillips, A. M. Valente-Feliciano, “High etching rates of bulk Nb in Ar/Cl2 microwave discharge,” J. Vac. Sci. Technol. A. 27 (2), 301 (2009)

16. Flat Samples - Surface Roughness As received BCP 1:1:2, 20 mm RMS=254nm Ra=210 nm RMS =286nm Ra=215nm BCP + PE RMS =215nm Ra=169 nm 3-steps PE ,120 mm RMS=234nm Ra=174 nm 120 mm AFM scans (50 μm 50 μm)

17. 3-step process 1. Pure Ar removes physisorbed gasses and organic residues from Nb surface without damaging surface Time 30 min Total flow 150 sccm Input power density 2.08 W/cm3 Pressure 500 mTorr Etching rate 0nm/min 2. 3 Vol% Cl2 in Ar removes surface necessary for cavity production Time 120 min Total flow 198.6 sccm Input power density 2.08 W/cm3 Pressure 550 mTorr Etching rate 1 mm/min removes ~120 mm of surface in 2 h 3. 1.5 Vol% Cl2 in Ar removes surface under conditions more favorable for surface smoothening Time 240 min Total flow 348.6 sccm Input power density 1.4 W/cm3 Pressure 1250 mTorr Etching rate 0.5 mm/min

19. Discharge using Cl2 as the reactive gas.

20. Nb etching rate depends on Cl2 reactive gas concentration and discharge parameters: input power density and pressure in reaction chamber.

21. Surface composition analyses (EDS, XPS) show that no impurities have been introduced into Nb during microwave discharge treatment.

22. Developed 3-step process.

24. Single Cell Sample Cavity Sample and holder Single Cell Cavity for Sample Etching New Electrode

25. Single Cell Cavity Process Asymmetric Discharge The scaling of the voltage drop in the plasma sheath with the surface area of the electrode : Optical Intensity Time waveforms of spectral line intensity at both sheaths in the discharge

26. Single Cell Cavity Electrode Movable multiple optical fiber system attached with spectrometer used for tomographic study of plasma inside the cavity. Spectrometer

27. Sample cavity setup with fiber optics

29. Tomography of the Plasma in the Cavity Stainless Steel Tube Ceramic Tube Optical Fibre 1.4 mm diameter 1.6 mm diameter 1.0 mm length of langmuir probe Movable multiple optical fiber system combined with electrical probe attached to the spectrometer will be used for tomographic study of plasma inside the cavity.

30. Next Steps Choose an optimal power supply frequency between RF (50-200 MHz) and MW (2.45 GHz.) Investigate the plasma etching behavior on samples placed on actual geometry of the single cell cavity (plasma distribution, roughness uniformity.. .) Plasma etching of single cell cavities and RF performance measurements.

31. Conclusion The RF performance is the single feature that remains to be compared to the “wet” process, since all other characteristics of the “dry etching ” technology, such as etching rates, surface roughness, low cost, and non-HF feature, have been demonstrated comparable to the currently used technologies. Final surface modifications (final oxidation, nitridation…) can be done in the same process cycle with the plasma etching process. The geometry of the inner surface of the cavity implies that the plasma discharge has to be asymmetric. In order for the asymmetric discharge to be effective, the lower sheath voltage at the treated surface (large area, undriven electrode) has to be at least equal or higher to the plasma floating potential at every point of the surface.

32. Surface Roughness Figure 13. Comparison of surface micrographs taken with KH-3000 digital microscope with magnification 10×350: (a) an untreated sample; (b) BCP sample; (c) plasma-etched (a) (a) (b) Surface micrographs obtained with scanning electron microscope: (a) sample treated with BCP technique – magnification 500x, (b) plasma treated sample – magnification 1500x. Black lines indicate distance of 10 mm.

33. Optical Emission Spectroscopy Intensity of Cl2 continua around 257 nm . Intensity of Cl2 continua around 308 nm as function of input power density in Ar/Cl2 discharge.

34. Flat Samples Etching Rate Dependence on Discharge Parameters M. Rašković, S. Popović, J. Upadhyay, and L. Vušković, L. Phillips, A. M. Valente-Feliciano, “High etching rates of bulk Nb in Ar/Cl2 microwave discharge,” submitted to J. Vac. Sci. Technol. A.

36. In this case plasma exists as an electronegative core (n+ ≈ n-) and electropositive halo (n+ ≈ ne).

38. Single Cell Cavity-Data from Flat SampleSomeSpectroscopic Results P = 1.3 W/cm3, Ar: 97%,, 3% Cl Excitation temperature jumps about 3 min after the discharge inception. Intensity of Cl2 continua around 308 nm as function of input power density in Ar/Cl2 discharge. P = 1.3 W/cm3, Ar: 97%,, 3% Cl P = 1 Torr, Ar: 97% , Cl2: 3% Excitation temperature in the absence of Nb sample is practically constant in the applied power density range. Nb I lines become prominent only after about three minutes from start.

42. Work done in collaboration with S. Popovic and L. Vuskovic Old Dominion University, Norfolk , Virginia A.– M. Valente –Feliciano and L. Phillips JLab, Newport News , Virginia